Continuing work using calcium stable isotopes as tracers in human subjects for the determination of absorption of dietary calcium and its subsequent distribution and excretion has led to a number of clinically significant findings. 1) Endogenous fecal excretion of calcium was determined in five healthy children ages 3-14 years. It averaged slightly lower than the urinary calcium excretion rate. The results are consistent with previously reported values for healthy adults and are about 5 times lower than the value we have previously determined in premature infants. 2) Developmental changes in calcium distribution dynamics were determined and compared in 10 healthy children, ages 10 months to 14 years, and in 3 adult women, ages 23-33 years. The size of the total exchangeable calcium pool and the rate of bone accretion was determined from a sum of exponentials fit of observed intravenous tracer dilution in plasma and urine after correcting for fecal and urinary losses. Accretion was significantly greater in children than adults. In the 4 prepubertal young adolescents, the total exchangeable pool was twice as great as it was in the adults. 3) Calcium absorption and distribution dynamics were studied in 7 children with juvenile rheumatoid arthritis, JRA. Dietary calcium intake was at normal levels as were absorption and bone accretion, compared to age-matched controls. Urinary calcium was elevated by as much as 4 fold. The normal rates of bone accretion and absorption coupled with elevated urinary excretion suggest that bone resorption rather than hyperabsorption occurs in JRA patients with hypercalcuria.